1. Field of the Invention
The present invention relates to the testing of integrated circuit chips and wafers and more particularly concerns a test probe that is self-leveling and appropriately tensioned.
2. Description of Related Art
Integrated circuit chips are manufactured with large numbers of identical circuits on a single wafer which ultimately are separately cut from the wafer for use. It is desirable to test each circuit individually to determine whether or not it functions as intended before separating it from the wafer. Additional testing of separated circuits may be desired at various stages in assembly of the circuit in the finished apparatus. Further testing may be accomplished after packaging a circuit, after placing it in a multi-chip module and after disassembly of a multi-chip module to identify inoperable circuits.
Conventional testing employs a probe card that is provided with a large number of small tungsten blades or needles that are mechanically and electrically connected to a circuit board and act as test probe contacts. Electrical leads extend from the contacts to the outer edge of the board for connecting the probe card to test circuitry. In use the blades or needles are moved into engagement with the pads of an integrated circuit. The motion has to be such that there is a slight scrubbing action that is required for breaking through oxidation coating that often covers an aluminum integrated circuit chip pad. Thus, blades or needles are frequently positioned at an angle so that they will effectively slide along or scrub a surface of the pad to break the oxide coating. This provides an electrical connection so that signals can be read to determine integrity of the circuit on the chip.
Ends of the test needles or blades must all fall in the same plane in order to assure that each one makes electrical contact with a pad of the integrated circuit. This is accomplished by bending the blades or needles after they are mounted on the probe card, which is laborious, time consuming and expensive. Even after such adjustment, the blades or needles tend to creep back toward their original position so that their adjusted locations are lost. This loss of adjustment also comes about from the pressure of the needles against the chips, aggravated by the scrubbing action used to assure penetration of an oxide coating. As a result, constant maintenance is necessary or the probe cards will not perform their intended function. Even when in proper adjustment the needles cannot compensate for significant differences in the heights of the contact pads on the integrated circuit chips being tested. The close spacing necessary for testing some chips cannot be achieved with conventional needle contacts. The needles may also apply excessive force against the chip so as to damage the chips or their contract pads. This problem is greatly magnified by the fact that a single chip may require testing at different stages in its assembly into a finished module. In fact, some specifications will limit the number of times that a single chip can be tested in order to avoid excessive chip damage caused by the testing operation itself.
Improved testing arrangements are disclosed in a co-pending application, Ser. No. 606,676, filed Oct. 31, 1991, by John Pasiecznik, Jr. for Method and Apparatus for Testing Integrated Circuits (Attorney Docket Nos. PD-90358, 76-242-D), and in a co-pending application Ser. No. 752,422, filed Aug. 30, 1991 by Blake F. Woith and William R. Crumly for Rigid Flex Circuits With Raised Features as IC Test Probes (Attorney Docket Nos. PD-91297, 83-171-D). Both of these applications are assigned to the same assignee as that of this application, and both are incorporated herein by reference as though fully set forth.
In the above-identified application of John Pasiecznik, Jr. a flexible membrane is stretched across a rigid annular substrate and provided with raised features on one side which connect through conductive circuit traces to a probe card that is in turn connected to the test circuitry. During use of the membrane probe for testing, the membrane is distended by air pressure, and contact of the membrane with the integrated circuit under test deflects the membrane to ensure that raised contacts formed on the membrane are pressed against the pads of the integrated circuit to provide an electrical connection. Deflection of the membrane during testing creates a desired wiping action. Electrical traces are also formed on the membrane to electrically connect the raised probe test contacts to test circuitry.
It is important in the use of the membrane test probe that the membrane itself be properly tensioned and configured, because the membrane, which is effectively stretched across the inner area of a annular support, tends to sag or stretch in a haphazard manner. Moreover, because test probe contacts or contact pads on a device to be tested are not all of the same height, good electrical contact may not be accomplished between all test probe contacts and all contact pads of the device under test. Further, because of the curvature of the membrane, the plane of the group of probe test contacts carried by the membrane may not be precisely parallel to the plane of the contact pads of the device under test. Moreover, the test apparatus and the device under test are not necessarily precisely parallel with one another so that the plane of the probe test contacts may be at times slightly inclined to the plane of the contact pads of the device under test.
Accordingly, it is an object of the present invention to provide a membrane test probe that avoids or minimizes above mentioned problems.